This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Puberty is characterized by a significant increase in growth velocity, changes in body composition and the appearance of striking somatic changes. Those changes have a potential to significantly affect the pharmacokinetics (PK) and pharmacodynamics (PD) of antiretroviral (ARV) drugs, particularly those with a narrow therapeutic window. Among the ARV drugs Efavirenz (EFV) has the smallest window as well as a very low genetic barrier for the development of viral resistance and treatment failure. EFV is a substrate of the cytochrome (CYP) P450 metabolic pathway (primarily CYP2B6), and in vitro data of EFV metabolism suggest that 8-hydroxylation of EFV is a specific marker of CYP2B6 activity. We propose to evaluate the effect of physical and hormonal changes during puberty on biotransformation and clearance of EFV. We hypothesize that the developmental changes during puberty affect the disposition of EFV and have the potential to alter the clinical outcome of EFV based therapy in HIV-infected children and adolescents by influencing the dose-plasma concentration relationship. Study Aim1: Evaluate the relationship of developmental changes during puberty to clearance (CL/F) of EFV through a cross-sectional study of 44 children and adolescents in Tanner stages I-IV. Study Aim 2: Evaluate the relationship of developmental changes during puberty (measured by somatic growth and sexual maturation) to CYP2B6 activity (determined by the formation clearance of EFV to its metabolite 8-hydroxyefavirenz) in the cross-sectional cohort of 44 children in study aim 1 and a longitudinal prospective cohort of 22 children in Tanner stages I-II. Study Aim 3: Explore the effect of developmental changes in CYP2B6 activity on the clinical outcome (measured by HIV-RNA viral load and CD4+ cell count) and toxicity (measured by CNS adverse events) of EFV based therapy in HIV-infected children and adolescents in a prospective cohort of 22 children in Tanner stages I-II.